Dielectric material



g- 23, 1955 E. B. BAKER 2,716,190

DIELECTRIC MATERIAL Filed Feb. 25, 1951 Microwave Lenses: magnesiummeza/ parflc/es d/Lgoerseafn ,oaxs/yrene.

Fry 5 19 4 11 i5 5 15 ill 5 I" Half Wave coating of Magnesium samecomponenfs w/Z/z o /eleczrfc consianl ba/f INVENTOR' Edward 84 Bakerzf/ml ofboa y BY ATTORNEYS parlic/es /'/2 polystyrene Ii United Statesatent DIELECTRIC MATERIAL Edward B. Baker, Midland, Mich., assignor toThe Dow Chemical Company, Midland, Mich., a corporation of DelawareApplication February 23, 1951, Serial N 0. 212,365

6 laims. (Cl. Mil-33.63)

This invention relates to an improved dielectric material, and, morespecifically, to a dielectric material consisting essentially of smallmetal particles dispersed in a polymerized plastic matrix.

Polystyrene-metal compositions having the electric properties requiredfor such diverse uses as special capacitors or resistors for highfrequency transmission lines, and lenses used in microwave beamtransmission work, have heretobefore been produced. For example, U. S.Patent 2,403,657 issued to Harvey on July 9, 1946, teaches a dielectricmaterial consisting of copper particles embedded in polystyrene. Such amaterial is said to be distinguished from polystyrene in that the ratioof reactance to resistance is constant or increases slightly withincrease of frequency in the range from to 400 megacycles. As a result,it is suitable for use in capacitors or resistors designed to operateover such a range. Another type of special dielectric material has beenproduced by placing metal strips in a predetermined pattern in slottedfoamed polystyrene sheets. The sheets so fitted with metallic strips arethen stacked to form a solid foamed polystyrene body having metal stripsembedded therein in a desired pattern. Lenses used in microwavetransmission have been produced in this way. A further discussion ofthis latter special dielectric material can be found in The Bell SystemTechnical Journal, vol. XXV H, No. 2, page 210 (1948).

The present invention is based upon the discovery of a simple,inexpensive dielectric material having a high dielectric constant and alow power factor so that it is suitable for the manufacture of microwavelenses.

The principal object of the present invention is to provide an improveddielectric material.

A further object is to provide an improved microwave lens that can beproduced at a fraction of the cost of previous lenses.

Still another object of the invention is to provide an improvedmicrowave transmission system.

More specific objects and advantages are apparent from the descriptionand drawings, which illustrate and disclose, but are not to be construedas limiting the inventron.

According to the invention an improved dielectric material is provided.This dielectric consists essentially of metal particles of size smallerthan 100 mesh, U. S. Sieve Series, dispersed in a polymerized matrix ofpolyethylene, a silicone rubber, of a polymerized 2-aryl alkene. Themetal particles are aluminum or magnesium, atomic numbers 12 and 13, andmust have an electric-insulating surface coating. If the matrix is apolymerized 2-aryl alkene, the alkene has not more than three carbonatoms, and the aryl radical is mononuclear, contains not more than tencarbon atoms, has not more than two substituents, and has no substituentother than chlorine and alkyl. The matrix of dielectric materials of theinvention can be either a solid matrix of the type produced by ordinarymolding techniques, or can be an expanded matrix of the type producedaccording to the teachings of U. S.

Patent 2,450,436 to Mclntire. The term silicone rubher is used in itsusual sense, namely to include vulcanized silicone resins, usuallycomprising methyl and phenyl silicones. All mesh sizes refer to the U.S. Sieve Series.

The magnesium or aluminum particles that are used to produce thecompositions of the invention should be in the shape of rods, spheres orplates, the two first-named shapes being preferred, and in the ordernamed. When the particles are rods it is ordinarily preferred that theyhave a maximum length not greater than about 0.1 inch and a diameter notgreater than about 0.01 inch, and the smaller the diameter thereof thebetter are the electric characteristics of the composition; this latteris true so long as the rods are of sufiicient size that they are notdeformed in processing. When the particles are spheres or plates it isusually preferred that substantially all of them be finer than mesh, andthat at least a substantial portion thereof be finer than 200 mesh.Particles as fine as can be produced are effective, e. g., evenparticles smaller than 325 mesh.

When the particles are magnesium no particular treatment is required toform the electric-insulating coating. The action of air on magnesiumparticles, even at room temperature, is suflicient to produce an oxidefilm satisfactory as an insulation. The oxide coating that forms onaluminum exposed to air is not sufficient to effect the requiredinsulation. Accordingly, when the particles are aluminum it isadvantageous that the insulating surface coating be appliedartificially, by some such method as anodizing the aluminum, or coatingthe particles with such an insulating material as a silicone. It will bereadily seen from the foregoing remarks that the preferredelectric-insulating coating is an oxide of the coated metal, and thatmagnesium is the preferred metal for use in producing compositions ofthe invention because special treatment thereof is not required.

It is usually preferred that the volume ratio of metal particles tototal volume of dielectric material be from 0.52100 to 40:100. The exactproportion of metal particles used is determined by the characteristicsrequired of the dielectric material. For example, when a non-expandeddielectric material is used to produce a microwave lens, it is desiredthat the material have a high dielectric constant. Such a characteristicis achieved by using a large ratio of metalparticles, e. g., from 30 to40 volumes per 100 volumes of dielectric. When the dielectric materialconsists of metallic particles dispersed in a foamed matrix it isusually preferred that the volume ratio of metallic particles todielectric material be from 0.5: 100 to 10:100. A larger proportion ofmetallic particles can be employed to produce a dielectric with a foamedmatrix or a smaller proportion than 30 volumes per 100 volumes ofdielectric with a non-expanded matrix, if desired.

Preferred dielectric materials of the invention have matrices of apolymerized 2-aryl alkene of the genus hereinbefore defined. Mostdesirably, the matrix is polystyrene, polyalphamethyl styrene, or acopolymer of styrene and alphamethyl styrene.

Improved dielectric materials of the invention are readily produced byconventional methods. The metal particles and the polyethylene, siliconerubber, or polymerized 2-aryl alkene are mixed. A dielectric having anon-expanded matrix can then be produced from the mixture by any of theconventional molding techniques, or by extrusion. A dielectric materialhaving a foamed matrix is readily produced from such a mixture by addinga normally gaseous substance to a vessel containing the mixture andprocessing as described in U. S. Patent 2,450,436 to McIntire.

The invention may be more fully understood by reference to the drawings,in which Fig. l is a front view of a microwave lens of the invention;

Fig. 2 is a side view of the microwave lens of Fig. 1;

Fig. 3 is a front view of an alternative shape of a microwave lens ofthe invention;

Fig. 4 is a side view of the microwave lens of Fig. 3, in section alongthe line 44 in Fig. 3;

Fig. 5 is a schematic diagram showing a transmission system with whichmicrowave lenses according to the invention are used; and

Fig. 6 is a cross-section, similar to Fig. 4, of an alternative form oflens according to the invention.

Figs. 1 and 2 illustrate a lens constituting a frustum of a cube. Such alens is effective by virtue of variations in dielectric constantthroughout the lens. These variations can be achieved by compressionmolding the lens from a composite preform made from various samples ofpolystyrene containing differing proportions of aluminum or magnesium,for example, a series of nesting annular disks in which the proportionof aluminum or magnesium increases with increasing ring diameter. Thisis practicable only for lenses having a non-expanded matrix.

Figs. 3 and 4 illustrate a double convex lens of the invention. Such alens converges microwaves by virtue of variations in angle of incidence,and can be produced with an expanded or non-expanded matrix. In someinstances when a foamed matrix is produced, it may be advantageous tomold the foamed composition to approximately its final form before itsets. This can readily be accomplished by forming a gel from astyrene-magnesium composition in the manner described by McIntire in avessel that will serve as a mold of approximately the desired lensshape, and, after the desired cure, releasing the pressure therein. Thelenses so produced affect microwaves in a manner analogous to that inwhich optical lenses affect light waves, dielectric constant being thecounterpart of refractive index, so that microwave lens shapes aresimilar to optical lens shapes.

Fig. 5 illustrates schematically the transmission system with whichmicrowave lenses of the invention are used. The system consistsessentially of a transmitter 11, and a microwave receiver 12, with twomicrowave lenses 13 interposed between the transmitter and the receiver.The transmitter is essentially a point source of microwaves, and islocated at the focal point of the first lens, which then converts thewaves to a beam. The second lens converges the microwaves of the beam onthe receiver, which is located at the focal point of the second lens. Ahorn, not shown, is frequently employed to concentrate the signals onthe first lens. The transmitter, the lenses, and the receiver areusually mounted in towers. One lens is mounted close to the transmitterto form a beam; the other is mounted to converge the beam on thereceiver, which may be as many as miles from the transmitter.

A particularly advantageous dielectric material of the invention isproduced when aluminum or magnesium rods are mixed with the desiredmatrix material, and the dielectric is produced by compression moldingor extrusion of the resulting composition. Either of these two methodsof production results in the orientation of the rods in the matrix sothat their long dimension is parallel to one surface of the dielectricmaterial. This orientation causes the resulting dielectric material tobe selective in that its dielectric constant is much higher when anapplied E. M. F. has an electric vector parallel to the surface of thematerial than when an applied E. M. F. has no such vector. In practice,imcrowaves are so polarized before they pass through a lens that theirprincipal electric vector lies in a plane perpendicular to the directionof wave transmission. Accordingly, dielectric materials of the inventionhaving rods oriented in a plane parallel to the fiat surfaces of thematerial are particularly effective materials for the production ofmicrowave lenses, and lenses produced from such a dielectric can besubstantially thinner for a given focal length than can microwave lenseshaving a foamed or solid matrix containing unoriented aluminum ormagnesium particles.

Because the dielectric constant of dielectric materials of the inventioncan be varied by changing the proportion of aluminum or magnesiumincorporated therein, it is possible to minimize wave reflection on thesurface of lenses of the invention by applying to the surface thereof afilm of a dielectric material of the invention having a dielectricconstant about one-half that of the main body of the lens. An especiallyadvantageous technique in this connection involves the application of afilm having a thickness of about one-half of the wave length of themicrowave with which the lens is to be employed. In this way, reflectionis minimized, and reflection from the surface of the coating tends tocancel reflection from the surface of the lens itself so thatinterference with wave transmission is minimized (see Fig. 6).

The following examples illustrate and disclose, but are not to beconstruted as limiting, the invention:

EXAMPLE 1 Different magnesium powders were incorporated in polystyreneto produce new dielectric materials. The samples of magnesium wereproduced as follows:

Sample No. ].Magnesium metal, atomized by the method described in U. S.Patent 1,351,865, to Nicol, issued September 7, 1920, screened to removeall material coarser than 100 mesh;

Sam le No. 2.Same material, screened to remove everything coarser than200 mesh;

Sample No. 3.Fines not collected in the atomization process, but removedfrom the exhaust gases from this process in a cyclone separator,screened to remove everything coarser than 200 mesh;

Sample No. 4.Magnesium metal filings, mostly to mesh; and

Sample No. 5.Smaller than 200 mesh ground magnesium metal.

Dielectric materials were produced from each of the above magnesiumsamples by milling from one to three parts by weight of the appropriatemagnesium metal sample with three parts by weight of polystyrene todisperse the magnesium in the polystyrene, and compression molding adisc from the resulting mixture. This resulted in a material containing16.9 to 33.8 volume per cent of magnesium. The dielectric materials soproduced had the properties set forth in Table 1 below.

Table I Fartl lstb fr Porer Factor erg 0 ercent Magnesium Sample No. FMetal to at 10,405

P 3 2 Cycles IO Cy- 10 Gy- Styrene eles cles If, for purposes ofcomparison, but not in accordance with the invention, copper powder ismixed with polystyrene, and small discs are molded from the resultingmixture, dielectric materials are produced which have higher dielectricconstants than polystyrene itself, but are far inferior to dielectricsof the invention. For example, if a composition containing 20 weight percent of copper powder is produced, it is found that the dielectricconstant in the range from 10 to 10 cycles per second is only about 3,although the per cent power factor is a maximum of 0.2 at 10 cycles persecond.

EXAMPLE 2 3. A microwave lens composed of a low-density dielectricmaterial consisting essentially of substantially rod-shaped particles ofmetal of atomic number 12 to 13 having an electric-insulating oxidesurface coating and having a length not greater than 0.1 inch and adiameter not greater than 0.01 inch dispersed in a foamed matrix of apolymerized 2-aryl alkene in which the alkene radical has not more thanthree carbon atoms and the aryl radical is mononuclear, contains notmore than ten carbon atoms, has not more than two subl n01} number thegram.s i Polystyrene grams of mag stituents, and has no substitutentother than chlorine nesium rods, c. c. of liquid propylene, temperatureand and alkyl i amp 3 i i i i thefdlelecltnc fi 4. A microwave lenscomposed of a dielectric ma- 1 eexpanthe ma f fil P g terial consistingessentially of substantially rod-shaped s gg e imwer ac or o 6 ma enawas 5 particles of metal of atomic number 12 to 13 having P 9911 anelectrically insulating oxide surface coating and hav- T able 2 G C O fCuring Cycle Grams rams of .0 Run N0. of Poly- Magne- Propyafigggistyrene sium 1 lane O C Time, a C Time,

' days days September 16, 1949, by Leontis and Pashak.

Expanded polystyrene itself has a dielectric constant of 1.1 to 1.2.

EXAMPLE 3 Other samples of dielectric materials were produced by theprocedure of Example 1, except that the magnesium was merely stirredinto powdered polystyrene, without milling, using rod-like magnesiumparticles produced as described in Example 2. The weight per cent ofmagnesium used, and the electrical properties of the final materials areset forth in Table 3, below.

1 At 10 cycles.

Microscopic examination of a sample of the dielectric material showedthe orientation of the magnesium rods in planes parallel to the surfaceof the dielectric. It is noted that dielectric constant was measuredwith the electric vector of the applied voltage at right angles to thesurface of the dielectric material in the tests reported above. As isdiscussed herein the dielectric constant is at a maximum toward anapplied voltage having an electric vector in a plane parallel to thesurface of such a dielectric material.

I claim:

1. A microwave lens composed of a low-density dielectric materialconsisting essentially of substantially rod-shaped magnesium particleshaving a length not greater than 0.1 inch and a diameter not greaterthan 0.01 inch dispersed in a foamed polystyrene matrix.

2. A microwave lens composed of a low-density dielectric materialconsisting essentially of substantially rod-shaped magnesium particleshaving a length not greater than 0.1 inch and a diameter not greaterthan 0.01 inch dispersed in a foamed polyalphamethyl styrene matrix.

ing a length not greater than 0.1 inch and a diameter not greater than0.01 inch dispersed in a matrix of a polymerized 2-aryl alkene in whichthe alkene radical has not more than three carbon atoms and the arylradical is mononuclear, contains not more than ten carbon atoms, has notmore than two substituents, and has no substituents other than chlorineand alkyl, the particles being oriented so that their long dimension isparallel to one surface of the dielectric material.

5. A microwave lens composed of a dielectric material consistingessentially of substantially rod-shaped particles of magnesium having alength not greater than 0.1 inch and a diameter not greater than 0.01inch dispersed in a polystyrene matrix, the particles being oriented sothat their long dimension is parallel to one surface of the dielectricmaterial.

6. A microwave lens composed of a dielectric material consistingessentially of substantially rod-shaped particles of magnesium having alength not greater than 0.1 inch and a diameter not greater than 0.01inch dispersed in a polystyrene matrix, the particles being oriented sothat their long dimension is parallel to one surface of the dielectricmaterial, said lens having a surface coating of dielectric materialformed of the same components as aforesaid but having a dielectricconstant about one-half that of the body of the lens, the thickness ofthe coating being about one-half the wavelength for which the lens isdesigned.

References Cited in the file of this patent UNITED STATES PATENTS1,947,112 Ruben Feb. 13, 1934 2,054,454 Theis et al. Sept. 15, 19362,202,380 Hollmann May 28, 1940 2,343,531 Buchholz Mar. 7, 19442,379,790 Dimmick July 3, 1945 2,415,352 Iams Feb. 4, 1947 2,508,479Wheeler May 23, 1950 2,547,416 Skellett Apr. 3, 1951 2,579,324 Kock Dec.18, 1951 FOREIGN PATENTS 406,267 Great Britain Feb. 15, 1934 793,574France Nov. 23, 1935

1. A MICROWAVE LENS COMPOSED OF A LOW-DENSITY DIELECTRIC MATERIALCONSISTING ESSENTIALLY OF SUBSTANTIALLY ROD-SHAPED MAGNESIUM PARTICLESHAVING A LENGTH NOT GREATER THAN 0.1 INCH AND A DIAMETER NOT GREATERTHAN 0.01 INCH DISPERSED IN A FOAMED POLYSTYRENE MATRIX.